This invention relates to a self-countersinking screw, particularly adapted for use with prepared lumber or composite wood having a veneer coating, such as Melamine™, Arborite™ and Formica™. It has further application to cementaceous products such as cement board. The screw uses shaped cutters arranged circumferentially around the peripheral underside of the head of the screw, which enable the screw head to penetrate into the surface of the substrate, at least flush with the surface.
Screws with serrations or notches on the underside of the screw head are known. For example, U.S. Pat. No. 5,433,569 (Fall et al) discloses a screw for use in thin metal plating wherein an annular flange has a series of serrated ridges extending radially from the screw axis across the underside of the flange. These serrations are intended to resist further turning of the screw once the flange has contacted the metal surface.
U.S. Pat. No. 6,616,391 (Druschel) discloses a screw for use in plastic composite lumber wherein the screw head has an annular lip on the underside, the lip including a series of incised notches (teeth) to aid in penetrating the surface of plastic lumber and to resist backing out of the screw.
Canadian Patent 2,198,832 (Walther) discloses a self-countersinking screw with radiating cutter edges and pocket recesses on the underside of the screw head.
A problem with many existing screws arises where screws are driven directly into a plastic veneered substrate without utilizing a pilot hole. The screw heads tend to crush through the plastic veneer, causing cracking and raising a dimple in the veneered surface. In thinner plastic film veneers, torsion from the underside of the screw head tends to tear the film. When prior screws are driven into cement board, it has been extremely difficult to bury the head of the screw below the surface of the cement board, unless a pilot countersunk hole is utilized. Such additional step increases costs and time of installation.